Is there a thread-safe way to print in Perl?

Question

I currently have a script that kicks off threads to perform various actions on several directories. A snippet of my script is: ``` #main sub BuildInit { my $actionStr = ""; my $compStr = ""; my @component_dirs; my @compToBeBuilt; foreach my $comp (@compList) { @component_dirs = GetDirs($comp); #populates @component_dirs } print "Printing Action List: @actionList "; #--------------------------------------- #---- Setup Worker Threads ---------- for ( 1 .. NUM_WORKERS ) { async { while ( defined( my $job = $q->dequeue() ) ) { worker($job); } }; } #----------------------------------- #---- Enqueue The Work ---------- for my $action (@actionList) { my $sem = Thread::Semaphore->new(0); $q->enqueue( [ $_, $action, $sem ] ) for @component_dirs; $sem->down( scalar @component_dirs ); print " ------>> Waiting for prior actions to finish up... <<------ "; } # Nothing more to do - notify the Queue that we're not adding anything else $q->end(); $_->join() for threads->list(); return 0; } #worker…

Accepted Answer

To ensure your output isn't interrupted, access to STDOUT and STDERR must be mutually exclusive. That means that between the time a thread starts printing and finishes printing, no other thread can be allowed to print. This can be done using Thread::Semaphore[1].

Capturing the output and printing it all at once allows you to reduce the amount of time a thread holds a lock. If you don't do that, you'll effectively make your system single-threaded system as each thread attempts lock STDOUT and STDERR while one thread runs.

Other options include:

- Using a different output file for each thread.

- Prepending a job id to each line of output so the output can be sorted later.

In both of those cases, you only need to lock it for a very short time span.

```
# Once
my $mutex = Thread::Semaphore->new();  # Shared by all threads.

# When you want to print.
$mutex->down();
print ...;
STDOUT->flush();
STDERR->flush();
$mutex->up();
```

or

```
# Once
my $mutex = Thread::Semaphore->new();  # Shared by all threads.
STDOUT->autoflush();
STDERR->autoflush();

# When you want to print.
$mutex->down();
print ...;
$mutex->up();
```

Is there a thread-safe way to print in Perl?

Solution

Problem